Conventionally, a glow plug that has been widely used is configured such that a rodlike ceramic heater is disposed in a distal end portion of a tubular metallic shell with a distal end portion of the ceramic heater projecting from the distal end portion of the metallic shell. Current supplied to the ceramic heater flows along a route running through: a metallic rod disposed at a rear end portion of the metallic shell and connected to a power supply; a metallic lead element that connects the metallic rod and the ceramic heater; a first metallic fitting member; an electric conductor (including a conductive portion, a resistance-heating element, and a conductive portion) of the ceramic heater; a second metallic fitting member; the metallic shell; and an engine head. In a conventional glow plug, in order to suppress an increase in contact resistance, a ceramic heater and a metallic lead element are connected, for example, as follows: metal plating is performed on a portion of a conductive portion exposed from the ceramic heater (hereinafter, the portion may be referred to merely as an “exposed portion”), the conductive portion being connected to the resistance-heating element; and the ceramic heater is fitted into a metallic fitting member, to which the metallic lead element is connected, such that the exposed portion comes into contact with the metallic fitting member (see Japanese Patent Application Laid-Open (kokai) No. 61-175415).
According to a method for obtaining the above-mentioned plated exposed portion of the conductive portion, the conductive portion is first plated and is then embedded in the ceramic heater. However, this method involves a potential instability in electrical connection, since plating on the exposed portion is potentially scraped or exfoliated during a finish grinding process for the ceramic heater or a process of joining, for example, the ceramic heater and the metallic fitting members, or a ceramic heater assembly and the metallic shell. A conceivable method for avoiding this problem is to plate only the exposed portion of the conductive portion after embedment of the conductive portion. However, in order to perform such plating, the entire ceramic heater except the exposed portion is masked against plating and is then subjected to plating. Specifically, the entire ceramic heater is dipped in a plating solution. Studies conducted by the present inventors have revealed that such dipping in the plating solution damages ceramic, thereby impairing durability of the ceramic heater.
In some cases, the above-mentioned conductive portion of a ceramic heater contains W and/or Mo. Addition of such an element(s) imparts, to the conductive portion of the ceramic heater, appropriate resistance and a coefficient of thermal expansion near that of a ceramic substrate, which surrounds the conductive portion, so that the glow plug can have sufficiently high reliability. However, in the conductive portion that contains W and/or Mo, in some cases an oxide film may be formed on the surface of its exposed portion due to heat generated by the ceramic heater. As a result, even when the exposed portion of the conductive portion is plated with metal, contact resistance between the metallic fitting member and the exposed portion potentially increases.